H2SO4 = 98-08 (98)
This is sold in various degrees of concentration and purity: Chamber
acid of 50-53° Baume, D = 1-53-1-58, with 62^67% H2S04; acid of 60°
Baume, D = 171, with 78% H2S04; ordinary English acid of 66° Baume,
D = 1-84, with 93-96% ; extra concentrated, with 96-98% and the mono-
hydrate, with 99-5%. As regards the purity, there is more or less impure
commercial acid, containing more especially lead, iron, arsenic, sulphurous
acid, nitrous products and organic matter as impurities, and the pure or
puriss. acids which should not contain the above or other foreign substances.
1. Fixed Residue.—10 c.c. are evaporated and calcined in a platinum
dish and any residue analysed in the ordinary way, especially for heavy
and alkali metals.
2. Lead, Iron and other Heavy Metals.—i vol. of the acid is poured
carefully into 5 vols. of 90% alcohol: no heavy, white deposit (lead sulphate]
should be formed, even after some hours.
5 c.c. are heated with a few drops of nitric acid, allowed to cool,
diluted with water and tested with potassium thiocyanate : no red coloration
(iron) should be observed.
i c.c., diluted with 20 c.c. of water, should give no brown coloration
with hydrogen sulphide (copper, lead), and after being made alkaline with
ammonia should not become brown (iron) or turbid (zinc).
3. Arsenic.—When used in the Marsh apparatus (see Flesh Foods,
Vol. II) for an hour, the acid should yield no arsenical mirror.
i c.c., diluted with 2 c.c. of. water and treated with 5 c.c. of Bet ten -
dorf's reagent (see p. 18), should not become coloured within an hour.
The quantitative determination is carried out as follows : 20 grams of the
acid are diluted with an equal volume of water and a current of sulphur di-
oxide passed through the liquid until the latter smells strongly of it. The
excess of sulphur dioxide is then expelled by means of carbon dioxide, a little
bicarbonate added and the arsenious acid titrated with N/io-iodine and
starch paste, i c.c. N/io-iodine = 0-00495 gram As203. If iron is
present in more than negligible amount, it must be eliminated before apply-
ing this method.
4. Selenium.—10 c.c. of the acid, diluted with 30 c.c. of water, are
treated with 20 c.c. of saturated sulphur dioxide solution : in presence of
selenium a reddish-yellow coloration appears, a slight red precipitate being
gradually deposited later (if the selenium is not too small in amount).
5. Ammonia.—2 c.c. of acid are diluted with about 30 c.c. of water,
then rendered alkaline with sodium hydroxide solution and tested with
Nessler solution (see Potable Waters).
6. Sulphurous Acid.—Starch paste is turned blue by a little dilute
iodine solution and the acid to be tested diluted and then added to the
starch, which is decolorised in presence of sulphurous acid. Or the acid
may be. diluted, a granule of zinc added and the liquid warmed : evolution
ol hydrogen sulphide is then tested for by means of lead acetate paper.
7. Nitrous Compounds, Nitric Acid.—Ferrous sulphate solution
A.C. 3. of Abater